Microsoft Windows 8 offers gesture-based passwords, in addition to traditional text-based passwords, in the hope that tracing a pattern on a familiar photograph is "secure but also a lot of fun to use."

It appears that picture gesture authentication (PGA) achieves only one of the two. Security researchers at Arizona State University and Delaware State University have found that Windows 8 picture passwords can be cracked with relative ease.

In a paper presented at the Usenix Conference earlier this month, "On the Security of Picture Gesture Authentication," Ziming Zhao, Gail-Joon Ahn and Jeong-Jin Seo from Arizona State, and Hongxin Hu from Delaware State, claim that their experimental model and attack framework allowed them to crack 48% of passwords for previously unseen pictures in one dataset and 24% in another.

This is with 219 guesses in a password space of 230 possibilities. Within the Windows 8 limit of five login attempts, the success rate is less: 216 out of 10,000 gesture passwords in one data set and 94 of 10,000 in the other one. The success rate improved with additional training data. Using a purely automated attack without supporting information, 0.9% of passwords could be cracked within five guesses.

Though that may not seem like a significant vulnerability, the fact remains that gesture-based passwords aren't as secure as Microsoft had hoped. In an email, Ahn said he expected the results could be improved with a larger training set and stronger picture categorization and computer vision techniques.

Setting up a gesture-based password involves choosing a photo from one's Picture Library folder and drawing three points on the image. The system accepts taps, lines and circles. Windows 8 subdivides the image into a 100 x 100 grid and stores the input points as grid coordinates.

Unfortunately, users aren't very good at selecting random points on their images; they tend to pick common points of interest, such as eyes, faces or discrete objects. As a result, passwords derived from this constrained set have much less variability than randomly generated passwords. So they're easier to crack.

Ahn says you only need to look at Microsoft's Windows 8 ads, which show users selecting obvious points of interest to form PGA passwords, to see that Microsoft's approach needs improvement.

The research paper suggests that Microsoft implement a picture-password-strength meter, similar to systems that prevent people from choosing weak text-based passwords. It also suggests that Microsoft integrate the researchers' PGA attack framework to inform users of the potential number of guesses it would take to access their system.